U.S. patent application number 13/489531 was filed with the patent office on 2013-12-12 for compact spectrometer for remote hydrocarbon detection.
This patent application is currently assigned to RAYTHEON COMPANY. The applicant listed for this patent is John F. Silny. Invention is credited to John F. Silny.
Application Number | 20130327942 13/489531 |
Document ID | / |
Family ID | 46679149 |
Filed Date | 2013-12-12 |
United States Patent
Application |
20130327942 |
Kind Code |
A1 |
Silny; John F. |
December 12, 2013 |
COMPACT SPECTROMETER FOR REMOTE HYDROCARBON DETECTION
Abstract
A multi-band imaging spectrometer and method of remote
hydrocarbon gas detection using the spectrometer. One example of
the multi-band imaging spectrometer includes a front objective
optical system, and an optical spectrometer sub-system including a
diffraction grating, the optical spectrometer sub-system configured
to receive and collimate an input beam from the objective optical
system to provide a collimated beam at the diffraction grating, the
diffraction grating configured to disperse the collimated beam into
at least two spectral bands. The spectrometer also includes a
single entrance slit positioned between the objective optical
system and the optical spectrometer sub-system and configured to
direct the input beam from the objective optical system to the
optical spectrometer sub-system, and a single focal plane array
optically coupled to the diffraction grating and configured to
produce an image from the at least two spectral bands.
Inventors: |
Silny; John F.; (Los
Angeles, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Silny; John F. |
Los Angeles |
CA |
US |
|
|
Assignee: |
RAYTHEON COMPANY
Waltham
MA
|
Family ID: |
46679149 |
Appl. No.: |
13/489531 |
Filed: |
June 6, 2012 |
Current U.S.
Class: |
250/339.02 ;
356/328 |
Current CPC
Class: |
G01J 3/2823 20130101;
G01N 21/3504 20130101; G01N 2021/1795 20130101; G01J 3/2803
20130101; G01J 3/18 20130101; G01J 3/0256 20130101; G01J 3/0294
20130101 |
Class at
Publication: |
250/339.02 ;
356/328 |
International
Class: |
G01J 3/42 20060101
G01J003/42 |
Claims
1. A multi-band imaging spectrometer comprising: an objective
optical system; an optical spectrometer sub-system including a
diffraction grating, the optical spectrometer sub-system configured
to receive and collimate an input beam from the objective optical
system to provide a collimated beam at the diffraction grating, the
diffraction grating configured to disperse the collimated beam into
at least two spectral bands; a single entrance slit positioned
between the objective optical system and the optical spectrometer
sub-system and configured to direct the input beam from the
objective optical system to the optical spectrometer sub-system;
and a single focal plane array optically coupled to the diffraction
grating and configured to receive the at least two spectral bands
and to produce an image from the at least two spectral bands.
2. The multi-band imaging spectrometer of claim 1, wherein the
objective optical system includes: a primary objective mirror of
positive optical power configured to reflect the input beam; a
secondary objective mirror of negative optical power configured to
receive the input beam from the primary objective mirror and to
reflect the input beam; and a third objective mirror of positive
optical power configured to receive the input beam from the
secondary objective mirror and to reflect the input beam to the
single entrance slit.
3. The multi-band imaging spectrometer of claim 1, wherein the
optical spectrometer sub-system includes a double-pass reflective
triplet.
4. The multi-band imaging spectrometer of claim 3, wherein the
single focal plane array is positioned at an image plane between
the single entrance slit and the double-pass reflective
triplet.
5. The multi-band imaging spectrometer of claim 4, wherein the at
least two spectral bands include the short-wavelength infrared band
and the mid-wavelength infrared spectral band.
6. The multi-band imaging spectrometer of claim 4, wherein the at
least two spectral bands include the short-wavelength infrared band
and the long-wavelength infrared spectral band.
7. The multi-band imaging spectrometer of claim 4, wherein the at
least two spectral bands include the mid-wavelength infrared band
and the long-wavelength infrared spectral band.
8. The multi-band imaging spectrometer of claim 7, wherein the at
least two spectral bands further includes the short-wavelength
infrared band.
9. The multi-band imaging spectrometer of claim 1, wherein the
single focal plane array includes at least one photo-detector
coupled to at least one read-out integrated circuit.
10. The multi-band imaging spectrometer of claim 1, wherein the
single focal plane array includes a monolithic photo-detector
coupled to a monolithic read-out integrated circuit.
11. The multi-band imaging spectrometer of claim 1, wherein the
single focal plane array includes at least two discrete
photo-detectors coupled to a monolithic read-out integrated
circuit.
12. The multi-band imaging spectrometer of claim 1, wherein the
single focal plane array includes a monolithic photo-detector
coupled to at least two discrete read-out integrated circuits.
13. The multi-band imaging spectrometer of claim 1, wherein the
single focal plane array includes at least two discrete
photo-detectors coupled to a corresponding at least two read-out
integrated circuits.
14. The multi-band imaging spectrometer of claim 1, wherein the
diffraction grating has a single blaze angle.
15. A method of remote hydrocarbon gas detection using an imaging
spectrometer comprising: directing an input light beam through a
single entrance slit; collimating the input light beam to provide a
collimated beam; dispersing the collimated beam into at least two
spectral bands, the spectral bands being separated in the spectral
dimension; directing the at least two spectral bands to an imaging
detector; and imaging and providing a spectral analysis of the at
least two spectral bands at the imaging detector.
16. The method of claim 15, wherein the method provides remote
detection of methane, the input light beam includes infrared light,
and wherein dispersing the collimated beam into the at least two
spectral bands includes dispersing the infrared light into at least
two of the short-wavelength infrared band, the mid-wavelength
infrared band, and the long-wavelength infrared band.
17. The method of claim 15, wherein dispersing the collimated beam
into the at least two spectral bands includes diffracting the
collimated beam with a diffraction grating to provide at least two
diffraction orders.
18. The method of claim 15, wherein directing the input light beam
through the single entrance slit includes reflecting the input
light beam with a reflective objective optical system to direct the
input light beam to the single entrance slit.
Description
BACKGROUND
[0001] Conventional systems for detecting methane use simple
"point" collectors that spectrally detect the presence of the
methane. These devices are limited in their ability to survey very
large areas while simultaneously providing adequate spectral and
radiometric sensitivity for high probability of detection with a
low rate of false alarms. Other approaches have used multiple
imaging spectrometers, each configured to cover a separate spectral
band. However, these systems typically have large size, weight and
power requirements due to multiple spectrometer optics and multiple
imaging detectors.
SUMMARY OF INVENTION
[0002] Aspects and embodiments are directed to a system and method
for remotely detecting a hydrocarbon gas, such as methane, from any
remote sensing platform, such as a ground-based, space-based or
airborne platform.
[0003] According to one embodiment, a multi-band imaging
spectrometer comprises an objective optical system, an optical
spectrometer sub-system including a diffraction grating, the
optical spectrometer sub-system configured to receive and collimate
an input beam from the objective optical system to provide a
collimated beam at the diffraction grating, the diffraction grating
configured to disperse the collimated beam into at least two
spectral bands, a single entrance slit positioned between the
objective optical system and the optical spectrometer sub-system
and configured to direct the input beam from the objective optical
system to the optical spectrometer sub-system, and a single focal
plane array optically coupled to the diffraction grating and
configured to receive the at least two spectral bands and to
produce an image from the at least two spectral bands.
[0004] In one example the objective optical system includes a
primary objective minor of positive optical power configured to
reflect the input beam, a secondary objective mirror of negative
optical power configured to receive the input beam from the primary
objective minor and to reflect the input beam, and a third
objective minor of positive optical power configured to receive the
input beam from the secondary objective mirror and to reflect the
input beam to the single entrance slit. In another example, the
optical spectrometer sub-system includes a double-pass reflective
triplet. The single focal plane array may be positioned at an image
plane between the single entrance slit and the double-pass
reflective triplet. In one example, the diffraction grating has a
single blaze angle. In one example, the at least two spectral bands
include the short-wavelength infrared band and the mid-wavelength
infrared spectral band. In another example, the at least two
spectral bands include the short-wavelength infrared band and the
long-wavelength infrared spectral band. In another example, the at
least two spectral bands include the mid-wavelength infrared band
and the long-wavelength infrared spectral band.
[0005] The single focal plane array may include at least one
photo-detector coupled to at least one read-out integrated circuit.
In one example, the single focal plane array includes a monolithic
photo-detector coupled to a monolithic read-out integrated circuit.
In another example, the single focal plane array includes at least
two discrete photo-detectors coupled to a monolithic read-out
integrated circuit. In another example, the single focal plane
array includes a monolithic photo-detector coupled to at least two
discrete read-out integrated circuits. In another example, the
single focal plane array includes at least two discrete
photo-detectors coupled to a corresponding at least two read-out
integrated circuits.
[0006] According to another embodiment, a method of remote
hydrocarbon gas detection using an imaging spectrometer comprises
directing an input light beam through a single entrance slit,
collimating the input light beam to provide a collimated beam,
dispersing the collimated beam into at least two spectral bands,
the spectral bands being separated in the spectral dimension,
directing the at least two spectral bands to an imaging detector,
and imaging and providing a spectral analysis of the at least two
spectral bands at the imaging detector.
[0007] In one example the method provides remote detection of
methane, the input light beam includes infrared light, and
dispersing the collimated beam into the at least two spectral bands
includes dispersing the infrared light into at least two of the
short-wavelength infrared band, the mid-wavelength infrared band,
and the long-wavelength infrared band. In one example, dispersing
the collimated beam into the at least two spectral bands includes
diffracting the collimated beam with a diffraction grating to
provide at least two diffraction orders. In another example,
directing the input light beam through the single entrance slit
includes reflecting the input light beam with a reflective
objective optical system to direct the input light beam to the
single entrance slit.
[0008] Still other aspects, embodiments, and advantages of these
exemplary aspects and embodiments are discussed in detail below.
Embodiments disclosed herein may be combined with other embodiments
in any manner consistent with at least one of the principles
disclosed herein, and references to "an embodiment," "some
embodiments," "an alternate embodiment," "various embodiments,"
"one embodiment" or the like are not necessarily mutually exclusive
and are intended to indicate that a particular feature, structure,
or characteristic described may be included in at least one
embodiment. The appearances of such terms herein are not
necessarily all referring to the same embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Various aspects of at least one embodiment are discussed
below with reference to the accompanying figures, which are not
intended to be drawn to scale. The figures are included to provide
illustration and a further understanding of the various aspects and
embodiments, and are incorporated in and constitute a part of this
specification, but are not intended as a definition of the limits
of the invention. In the figures, each identical or nearly
identical component that is illustrated in various figures is
represented by a like numeral. For purposes of clarity, not every
component may be labeled in every figure. In the figures:
[0010] FIG. 1 is a block diagram of one example of a photo-detector
configured for multi-spectral imaging;
[0011] FIG. 2 is a diagram of one example of a slit
re-formatter;
[0012] FIG. 3 is a block diagram of one example of an imaging
spectrometer system according to aspects of the invention;
[0013] FIG. 4 is a diagram of one example of an imaging
spectrometer system according to aspects of the invention;
[0014] FIG. 5A is a diagram of one example of a dual-band slit and
focal plane array configuration according to aspects of the
invention;
[0015] FIG. 5B is a diagram of another example of three-band slit
and focal plane array configuration according to aspects of the
invention;
[0016] FIG. 6A is a diagram of one example of a focal plane array
configuration according to aspects of the invention;
[0017] FIG. 6B is a diagram of another example of a focal plane
array configuration according to aspects of the invention;
[0018] FIG. 6C is a diagram of another example of a focal plane
array configuration according to aspects of the invention;
[0019] FIG. 6D is a diagram of another example of a focal plane
array configuration according to aspects of the invention;
[0020] FIG. 7A is a diagram of one example of a dual-band slit and
focal plane array configuration according to aspects of the
invention;
[0021] FIG. 7B is a diagram of another example of a dual-band slit
and focal plane array configuration according to aspects of the
invention;
[0022] FIG. 8A is a diagram of another example of a dual-band slit
and focal plane array configuration according to aspects of the
invention;
[0023] FIG. 8B is a diagram of another example of a dual-band slit
and focal plane array configuration according to aspects of the
invention;
[0024] FIG. 9A is a diagram of another example of a dual-band slit
and focal plane array configuration according to aspects of the
invention;
[0025] FIG. 9B is a diagram of another example of a dual-band slit
and focal plane array configuration according to aspects of the
invention;
[0026] FIG. 10A is a diagram of one example of a three-band slit
and focal plane array configuration according to aspects of the
invention; and
[0027] FIG. 10B is a diagram of another example of a three-band
slit and focal plane array configuration according to aspects of
the invention;
DETAILED DESCRIPTION
[0028] Aspects and embodiments are directed to a compact multi-band
imaging spectrometer, and to the use of a single such spectrometer
to cover multiple discrete spectral regions for hydrocarbon gas
detection. Imaging spectrometers are used to provide an image and
also a spectral analysis of the image in a selected wavelength band
of interest. These images and spectral analyses may be used to
remotely detect the presence of various chemical compounds,
including hydrocarbon gases, such as methane, ethane and/or
propane, for example. Methane gas has multiple spectral absorbance
features in the infrared wavelength band, including features in the
short-wavelength infrared (SWIR) spectral region from 2.1 to 2.6
micrometers (.mu.m), mid-wavelength infrared (MWIR) region from 3.1
to 3.5 .mu.m, and long-wavelength infrared (LWIR) region from 7.2
to 8.2 .mu.m. Remote sensors attempting to detect methane, or other
gases, from any appreciable distance (for example, airborne or
space-based sensors) will suffer from signal attenuation due to
atmospheric absorption. As a result, accounting for absorption in
the atmosphere, useful spectral features for methane detection
include SWIR features from 2.1 to 2.5 .mu.m, MWIR features from 3.3
to 3.5 .mu.m, and LWIR features from 7.7 to 8.2 .mu.m. Accordingly,
a spectrometer capable of collecting and analyzing all three of
these discrete spectral regions, with little to no band-to-band
mis-registration, may be desirable for methane detection.
Similarly, a compact spectrometer capable of collecting and
analyzing discrete spectral regions containing useful features
associated with other hydrocarbon gases may also be desirable.
[0029] Some approaches for multi-band detection have included using
a multi-band spectrometer with multiple entrance slits to spatially
separate the different spectral bands. For example, a three-band
spectrometer may use three entrance slits positioned between the
foreoptics and the focal plane array. FIG. 1 illustrates an example
of a focal plane array sensor 100 having three regions 110 each
responsive to a different wavelength band. In one example, for a
1024 pixel by 1024 pixel focal plane array 100, each region 110 has
a horizontal spatial extent of 160 pixels. The regions 110 are
spatially separated from one another by guard zones 120. In one
example, the guard zones have a horizontal spatial extent of 250
pixels. The focal plane array 100 may also be configured to include
edge tolerance regions 130, which may be 22 pixels across, for
example. As discussed above, three entrance slits 140 are
positioned above the detection regions 110, as shown in FIG. 1, to
direct incident light in the three different spectral bands to the
appropriate detection region 110.
[0030] Since in this example, the three discrete spectral bands are
offset from one another in the spatial dimension, a slit
re-formatter is used to co-align the fields of view of the regions
110 of the focal plane array 100. FIG. 2 illustrates an example of
a slit re-formatter 200. The slit re-formatter 200 includes
reflective optics (e.g., minors) 210 and dichroic beamsplitters 220
to split the incoming light 230 into the three discrete spectral
bands 240a, 240b, 240c and direct each spectral band to one of the
three entrance slits 140.
[0031] In contrast to the above-discussed approach, aspects and
embodiments are directed to a compact multi-band imaging
spectrometer in which different diffraction orders from a
diffraction grating are used to offset discrete spectral regions in
the spectral dimension, rather than the spatial dimension. A single
entrance slit may be used to provide spatial co-registration of the
spectra from all discrete spectral bands, and avoid band-to-band
mis-registration. As discussed in more detail below, a single blaze
angle diffraction grating may be used to provide high diffraction
efficiency for all selected discrete spectral bands. In addition,
multiple detector configurations may be implemented to optimize
system-level performance, such as the use of monolithic or discrete
detectors and/or read-out integrated circuits (ROICs) for the
multiple spectral regions. Embodiments of the imaging spectrometer
are scalable with multiple configurations to provide any two or all
three discrete infrared spectral bands suitable for methane
detection. Similarly, embodiments of the imaging spectrometer may
be configured to provide any or all of the discrete spectral bands
suitable for other hydrocarbon gas detection, such as detection of
ethane or propane, for example. Furthermore, certain embodiments
may use a variable number of spectral channels per discrete
spectral band to optimize spectral ranges and sampling intervals,
as discussed in more detail below.
[0032] It is to be appreciated that embodiments of the methods and
apparatuses discussed herein are not limited in application to the
details of construction and the arrangement of components set forth
in the following description or illustrated in the accompanying
drawings. The methods and apparatuses are capable of implementation
in other embodiments and of being practiced or of being carried out
in various ways. Examples of specific implementations are provided
herein for illustrative purposes only and are not intended to be
limiting. In particular, acts, elements and features discussed in
connection with any one or more embodiments are not intended to be
excluded from a similar role in any other embodiment.
[0033] Also, the phraseology and terminology used herein is for the
purpose of description and should not be regarded as limiting. Any
references to embodiments or elements or acts of the systems and
methods herein referred to in the singular may also embrace
embodiments including a plurality of these elements, and any
references in plural to any embodiment or element or act herein may
also embrace embodiments including only a single element. The use
herein of "including," "comprising," "having," "containing,"
"involving," and variations thereof is meant to encompass the items
listed thereafter and equivalents thereof as well as additional
items. References to "or" may be construed as inclusive so that any
terms described using "or" may indicate any of a single, more than
one, and all of the described terms.
[0034] Referring to FIG. 3, there is illustrated a block diagram of
one example of an imaging spectrometer system 300. According to one
embodiment, a single, compact multi-band imaging spectrometer
system 300 provides spatially co-located spectra covering multiple
discrete spectral bands. In one example, the multiple spectral
bands include infrared bands, such as the SWIR (e.g., 2.1 to 2.5
.mu.m), MWIR (e.g., 3.3 to 3.5 .mu.m) and LWIR (e.g., 7.7 to 8.2
.mu.m) bands discussed above useful for remote detection of
methane. According to one embodiment, the imaging spectrometer
system 300 uses a single entrance slit through which incident
electromagnetic radiation (also referred to as "white light") 310
is focused via an objective optical system (foreoptics) 320. The
spectrometer 330 includes optical elements to perform spectrometer
functions for collimation, dispersion, and imaging, as discussed
further below. In one example, the dispersive element is a
diffraction grating that disperses the incoming white light 310
into its constituent colors or bands, as illustrated in FIG. 3. By
using multiple diffraction orders, as discussed further below, a
set of discrete spectral bands may be imaged onto a focal plane at
detector 340 with an offset in the spectral dimension. Order
sorting filters may be used to block unwanted light for each
discrete spectral band. The detector 340 may be a panchromatic
imaging detector. In one example, detector 340 includes a single
focal plane array, which includes one or more photo-detectors and
associated read-out integrated circuitry, as discussed in more
detail below.
[0035] According to one embodiment the foreoptics 320 includes an
all-reflective objective, which may be made solely of minors and
with no lenses. The spectrometer 330 may include a reflective
triplet spectrometer, one example of which is disclosed in U.S.
Pat. No. 7,382,498. FIG. 4 illustrates an example of the foreoptics
320 in combination with the spectrometer 330, according to one
embodiment. In the illustrated example, the foreoptics 320 includes
a set of three minors including a primary objective minor 405 of
positive optical power, an objective secondary minor 410 of
negative optical power, and an objective tertiary minor 415 of
positive optical power. The foreoptics 320 direct the incoming
white light 310 to a single entrance slit 420. The spectrometer 330
may have a double-pass reflective form, which in the example
illustrated in FIG. 4 is a double-pass reflective triplet 430.
Accordingly, the panchromatic imaging detector 340 may be provided
on the input side (that is, the foreoptics side) of an image plane
425, where it will receive the double-passed output light beams
from the spectrometer 330. As discussed above, in one example, the
detector 340 is a single focal plane array.
[0036] In one embodiment, the reflective triplet 430 includes a
primary mirror 435 having a positive optical power, a secondary
mirror 440 having a negative optical power, and a tertiary minor
445 having a positive optical power. The three mirrors of the
spectrometer 330 collimate the incoming beam 450 received via the
entrance slit 420 and provide a collimated output beam 455 at a
dispersive element 460. As discussed above, in one example, the
dispersive element 460 is a diffraction grating. The dispersive
element 460 is configured and oriented to receive and disperse the
collimated output beam 455 and to direct the dispersed light 465
back through the double-pass reflective triplet to be incident on
the detector 340 at the image plane 425. The angular direction of
the dispersed light is determined by the spatial orientation of the
diffraction grating 460. The blaze angle and diffraction order(s)
of the diffraction grating determines the spectral dispersion of
the collimated output beam 455.
[0037] Thus, the reflective triplet of the spectrometer 330 is
referred to as a "double-pass" optical component because the light
beams travel through the reflective triplet 430, and are collimated
on the way to the dispersive element 460. Then, on the return path
from the dispersive element 460, the light travels through the
reflective triplet 430 and is imaged on the image plane 425.
Although not shown in FIG. 4, one or more fold minors may be used
to aid in directing the collimated output beam 455 and/or dispersed
light 465.
[0038] As discussed above, the focal plane array forming detector
340 may have numerous different configurations selected to optimize
system performance over the spectral bands of interest. FIG. 5A and
FIG. 5B illustrate two examples of entrance slit 420 and focal
plane array 500 combinations supported by embodiments the imaging
spectrometer. As discussed above, in one embodiment, the focal
plane array 500 is positioned behind a single entrance slit 420.
Referring to FIG. 5A, the focal plane array 500 may be configured
to support a two-band device, and may include regions 510, 520
where the two different spectral bands fall. For example, region
510 may be where the first spectral band falls and region 520 may
be where the second spectral band falls. A focal plane array is
composed of two parts, namely the detector and the ROIC. The
detector includes the light-sensitive material that receives light
and generates an electrical current. The ROIC is the electronic
circuit that captures the current during an exposure and then
transfers the signal to other electronics (e.g. for storage). As
discussed further below, the focal plane 500 may include any
combination of detector materials and/or ROICs to support
particular spectral bands of interest. Multiple configurations of
the focal plane array 500 may support various configurations of two
discrete spectral bands. For example, the regions 510 and 520 may
include material(s) selected to support an MWIR and SWIR
combination, an LWIR and MWIR configuration, or an LWIR and SWIR
configuration. Referring to FIG. 5B, in another example, the focal
plane array 500 is configured to support a three-band device and
therefore includes three regions 510, 520, 530 where the different
spectral bands fall, for example, MWIR (510), SWIR (520) and LWIR
(530). The result is a flexible imaging spectrometer architecture
that may be configured for optimal remote detection of a gas of
interest, such as methane, for example, or another hydrocarbon
gas.
[0039] According to one embodiment, the focal plane array 500 may
be further configurable in that it may include monolithic or
discrete detector materials and/or read-out integrated circuits
(ROICs). For example, referring to FIG. 6A, the focal plane array
500 may include a monolithic photo-detector 610 coupled to a
monolithic ROIC 620. This configuration provides a simple approach;
however, short-wavelength performance of the device may be
limited.
[0040] In another example, the focal plane array 500 includes
multiple discrete photo-detectors 630, 635 coupled to a monolithic
ROIC 620, as shown in FIG. 6B. This configuration is slightly more
complex than the example shown in FIG. 6A, and may require multiple
hybridizations. Hybridization refers to the process of coupling the
detector and ROIC parts of a focal plane array sensor. Accordingly,
in examples where multiple detector materials are used, multiple
hybridizations may be needed to couple each detector material to
the ROIC. However, this configuration may provide better dark
current performance for the short wavelength band(s), for example,
the SWIR band. Dark current is the constant response exhibited by a
receptor of radiation during periods when it is not actively being
exposed to light. For example, dark current may include the
relatively small electric current that flows through the
photosensitive detector(s) when no photons are entering the device,
and/or the constant response produced by a spectrochemical receptor
in the absence of radiation.
[0041] Referring to FIG. 6C, in another example, the focal plane
array 500 includes a monolithic photo-detector 610 coupled to
multiple discrete ROICs 640, 645. This configuration may be a more
complex approach, but may allow optimization of the unit cell
(focal plane array 500) for each spectral band.
[0042] FIG. 6D illustrates another example in which the focal plane
array 500 includes multiple discrete photo-detectors 630, 635
coupled to multiple discrete ROICs 640, 645. This approach may be
more complex, but may allow for optimized performance for several
parameters, including, for example, dark current, quantum
efficiency, unit cell type, full well (that is, the amount of
charge that the ROIC can store), and read noise, among others.
[0043] Thus, embodiments of the multi-band imaging spectrometer may
be configured and optimized in various ways to provide good
multi-band spectral performance for remote detection of a gas of
interest. A single entrance slit may be used to direct the incoming
electromagnetic radiation to the spectrometer components 330,
thereby avoiding band-to-band mis-registration. The radiation is
dispersed into its spectral components using a diffraction grating
and multiple diffraction orders to achieve spectral separation of
two or more spectral bands of interest. The detector 340 may be
configured in various ways, as discussed above, using different
materials and any of several photo-detector and ROIC configurations
to achieve good performance for all spectral bands.
[0044] The function and advantages of these and other embodiments
will be more fully understood from the following examples. The
examples are intended to be illustrative in nature and are not to
be considered as limiting the scope of the systems and methods
discussed herein. Table 1 below provides a summary of the following
four examples which are discussed in more detail below. For
simplicity, each of the four examples used a fixed number of
spectral channels, namely 256, per discrete spectral band (e.g. the
short wave infrared, SWIR). However, as discussed above, in other
embodiments, the number of channels per band may be varied to
optimize performance.
TABLE-US-00001 TABLE 1 Number Spectral Diffraction of Bands Bands
Orders Spectral Range SSI Blaze WL Avg. DE Example 1 2 SWIR 4 2.2
to 2.5 .mu.m 1.0 nm 9.7 .mu.m 81% MWIR 3 3.3 to 3.7 .mu.m 1.4 nm
88% Example 2 2 MWIR 5 3.3 to 3.6 .mu.m 1.0 nm 16.8 .mu.m 91% LWIR
2 7.7 to 8.3 .mu.m 2.5 nm 91% Example 3 2 SWIR 3 2.1 to 2.5 .mu.m
1.7 nm 9.6 .mu.m 84% LWIR 1 7.5 to 8.8 .mu.m 5.0 nm 83% Example 4 3
SWIR 3 2.3 to 2.6 .mu.m 1.0 nm 6.9 .mu.m 92% MWIR 2 3.0 to 3.5
.mu.m 1.5 nm 94% LWIR 1 7.7 to 8.5 .mu.m 3.0 nm 89%
[0045] In each example, the design should obtain separation (in the
spectral) direction of each band, such that the bands do not land
on the same part of the focal plane array (see FIG. 5A or 5B), and
a high diffraction efficiency (for high sensitivity) is desired.
The degrees of freedom in each design are the grating diffraction
orders, the minimum wavelength of one of the spectral bands (the
other is determined), and the spectral sampling interval of one of
the spectral bands (the other is determined).
Example 1
[0046] This example demonstrates performance of an embodiment of a
dual-band spectrometer configured to detect and analyze at least a
portion of each of the SWIR and MWIR spectral bands. In particular,
referring to FIGS. 7A and 7B, in this example, the focal plane
array 500 includes a first region 510 responsive to electromagnetic
radiation in the SWIR spectral range of approximately 2.2 to 2.5
.mu.m, and a second region 520 responsive to electromagnetic
radiation in the MWIR spectral range of approximately 3.3 to 3.7
.mu.m. The band layout in the focal plane array 500 may be as shown
in either FIG. 7A or FIG. 7B, and may depend on the grating
orientation. The focal plane array 500 may include one or more
material(s) suitable for photo-detection in the infrared band. One
example of a material commonly used for infrared detection is
mercury-cadmium-telluride (HgCdTe). By selecting an appropriate
ratio of Hg to Cd, the detector's spectral sensitivity can be
changed (for example, to be sensitive in the SWIR band versus the
MWIR band). Thus, the material may be written as
Hg.sub.1-xCd.sub.xTe, in which x is the factional doping
concentration of Cd and is selected to choose the correct spectral
response (e.g. x=0.3 for MWIR sensitivity).
[0047] In this example, the third diffraction order was used for
the MWIR spectral band, and the fourth diffraction order was used
for the SWIR spectral band. The spectral sampling interval (SSI)
was 1.4 nm for the third diffraction order and 1.05 nm for the
fourth diffraction order. Table 2 provides the center wavelengths
(CWL) detected by each of several example channels in the focal
plane array 500.
TABLE-US-00002 TABLE 2 Diffraction Order 3 Diffraction Order 4
Channel CWL (.mu.m) CWL (.mu.m) -256 2.94 2.21 -1 3.30 2.47 0 3.30
2.48 255 3.66 2.74
[0048] In this example, a diffraction grating (for dispersive
element 460) having a blaze wavelength of 9.7 .mu.m was used,
resulting in a peak diffraction efficiency (DE) of approximately
95% at the grating blaze wavelength. Table 1 above provides the
average diffraction efficiency for each spectral band. Table 3
below provides the diffraction efficiencies at certain wavelengths
within each of the SWIR and MWIR spectral bands for this example
configuration.
TABLE-US-00003 TABLE 3 Diffraction order 3 Diffraction order 4 CWL
(.mu.m) DE CWL (.mu.m) DE 3.30 94% 2.20 52% 3.35 92% 2.25 68% 3.40
89% 2.30 81% 3.45 85% 2.35 90% 3.50 80% 2.40 94% 2.45 95% 2.50
91%
Example 2
[0049] This example demonstrates performance of an embodiment of a
dual-band spectrometer configured to detect and analyze at least a
portion of each of the SWIR and MWIR spectral bands. In particular,
referring to FIGS. 8A and 8B, in this example, the focal plane
array 500 includes a first region 510 responsive to electromagnetic
radiation in the MWIR spectral range of approximately 3.3 to 3.6
.mu.m, and a second region 520 responsive to electromagnetic
radiation in the LWIR spectral range of approximately 7.7 to 8.3
.mu.m. The band layout in the focal plane array 500 may be as shown
in either FIG. 8A or FIG. 8B, and may depend on the grating
orientation.
[0050] In this example, the fifth diffraction order was used for
the MWIR spectral band, and the second diffraction order was used
for the LWIR spectral band. The spectral sampling interval (SSI)
was 2.5 nm for the second diffraction order and 1 nm for the fifth
diffraction order. Table 4 provides the center wavelengths (CWL)
detected by each of several example channels in the focal plane
array 500.
TABLE-US-00004 TABLE 4 Diffraction Order 2 Diffraction Order 5
Channel CWL (.mu.m) CWL (.mu.m) 0 7.700 3.080 255 8.338 3.335 256
8.340 3.336 511 8.978 3.591
[0051] In this example, a diffraction grating (for dispersive
element 460) having a blaze wavelength of 16.8 .mu.m was used,
resulting in a peak diffraction efficiency (DE) of approximately
95% at the grating blaze wavelength. Table 5 below provides the
diffraction efficiencies at certain wavelengths within each of the
MWIR and LWIR spectral bands for this example configuration.
TABLE-US-00005 TABLE 5 Diffraction order 2 Diffraction order 5 CWL
(.mu.m) DE CWL (.mu.m) DE 7.70 85% 3.30 92% 7.80 88% 3.35 95% 7.90
90% 3.40 94% 8.00 92% 3.45 90% 8.10 93% 3.50 84% 8.20 94% 8.30
95%
Example 3
[0052] This example demonstrates performance of an embodiment of a
dual-band spectrometer configured to detect and analyze at least a
portion of each of the SWIR and LWIR spectral bands. In particular,
referring to FIGS. 9A and 9B, in this example, the focal plane
array 500 includes a first region 510 responsive to electromagnetic
radiation in the SWIR spectral range of approximately 2.1 to 2.5
.mu.m, and a second region 520 responsive to electromagnetic
radiation in the LWIR spectral range of approximately 7.7 to 8.8
.mu.m. The band layout in the focal plane array 500 may be as shown
in either FIG. 9A or FIG. 9B, and may depend on the grating
orientation.
[0053] In this example, the third diffraction order was used for
the SWIR spectral band, and the first diffraction order was used
for the LWIR spectral band. The spectral sampling interval (SSI)
was 5 nm for the first diffraction order and 1.7 nm for the third
diffraction order. Table 6 provides the center wavelengths (CWL)
detected by each of several example channels in the focal plane
array 500.
TABLE-US-00006 TABLE 6 Diffraction Order 2 Diffraction Order 5
Channel CWL (.mu.m) CWL (.mu.m) -256 6.220 2.073 -1 7.495 2.498 0
7.500 2.500 255 8.775 2.925
[0054] In this example, a diffraction grating (for dispersive
element 460) having a blaze wavelength of 9.6 .mu.m was used,
resulting in a peak diffraction efficiency (DE) of approximately
95% at the grating blaze wavelength. Table 7 below provides the
diffraction efficiencies at certain wavelengths within each of the
MWIR and LWIR spectral bands for this example configuration.
TABLE-US-00007 TABLE 7 Diffraction order 2 Diffraction order 5 CWL
(.mu.m) DE CWL (.mu.m) DE 7.70 78% 3.30 92% 7.80 80% 3.35 88% 7.90
82% 3.40 84% 8.00 84% 3.45 80% 8.10 85% 3.50 74% 8.20 87% 8.30
88%
Example 4
[0055] This example demonstrates performance of an embodiment of a
three-band spectrometer configured to detect and analyze at least a
portion of each of the SWIR, MWIR and LWIR spectral bands. In
particular, referring to FIGS. 10A and 10B, in this example, the
focal plane array 500 includes a first region 510 responsive to
electromagnetic radiation in the MWIR spectral range of
approximately 3.0 to 3.5 .mu.m, a second region 520 responsive to
electromagnetic radiation in the SWIR spectral range of
approximately 2.3 to 2.6 .mu.m, and a third region 530 configured
to be responsive to radiation in the LWIR spectral range of
approximately 7.7 to 8.5 .mu.m. The band layout in the focal plane
array 500 may be as shown in either FIG. 10A or FIG. 10B, and may
depend on the grating orientation.
[0056] In this example, the third diffraction order was used for
the SWIR band, the second diffraction order was used for the MWIR
spectral band, and the first diffraction order was used for the
LWIR spectral band. The spectral sampling interval (SSI) was 3 nm
for the first diffraction order, 1.5 nm for the second diffraction
order, and 1 nm for the third diffraction order. Table 8 provides
the center wavelengths (CWL) detected by each of several example
channels in the focal plane array 500.
TABLE-US-00008 TABLE 8 Diffraction Order 1 Diffraction Order 2
Diffraction Order 3 Channel CWL (.mu.m) CWL (.mu.m) CWL (.mu.m)
-512 6.164 3.082 2.055 -257 6.929 3.465 2.310 -256 6.932 3.466
2.311 -1 7.697 3.849 2.566 0 7.700 3.850 2.567 255 8.465 4.233
2.822
[0057] In this example, a diffraction grating (for dispersive
element 460) having a blaze wavelength of 6.9 .mu.m was used,
resulting in a peak diffraction efficiency (DE) of approximately
95% at the grating blaze wavelength. Table 8 below provides the
diffraction efficiencies at certain wavelengths within each of the
SWIR, MWIR and LWIR spectral bands for this example
configuration.
TABLE-US-00009 TABLE 9 Diffraction order 1 Diffraction order 2
Diffraction Order 3 CWL (.mu.m) DE CWL (.mu.m) DE CWL (.mu.m) DE
7.70 92% 3.30 92% 2.20 89% 7.80 91% 3.35 94% 2.25 93% 7.90 90% 3.40
95% 2.30 95% 8.00 89% 3.45 95% 2.35 94% 8.10 89% 3.50 95% 2.40 91%
8.20 88% 8.30 87%
[0058] Having described above several aspects of at least one
embodiment, it is to be appreciated various alterations,
modifications, and improvements will readily occur to those skilled
in the art. Such alterations, modifications, and improvements are
intended to be part of this disclosure and are intended to be
within the scope of the invention. Accordingly, the foregoing
description and drawings are by way of example only, and the scope
of the invention should be determined from proper construction of
the appended claims, and their equivalents.
* * * * *